Image display devices such as cathode ray tube (CRT) display devices, liquid crystal displays (LCD), plasma displays (PDP), electroluminescence displays (ELD), electronic paper displays, tablet computers, and touch panels are generally provided with an optical layered body for antireflection on the outermost surface. Such optical layered bodies for antireflection suppress reflection of images and decrease the reflectivity by scattering or interference of light.
Known as one of optical layered bodies for antireflection is an antiglare film in which an antiglare layer with a surface roughness is formed on a transparent substrate. The surface roughness on the antiglare film scatters natural light, and therefore a decrease in visibility due to reflection of natural light and reflection of images can be prevented.
Such optical layered bodies are usually disposed at the outermost surfaces of image display devices, and therefore need to have hard coating properties in order to prevent the displays from being scratched during handling.
As conventional antiglare films, a film including a light-transmitting substrate and an antiglare layer formed by applying a resin containing a filler such as silicon dioxide (silica) on the surface of the substrate is known (refer to, for example, Patent Literatures 1 and 2).
Examples of the antiglare films include a film in which a surface roughness on an antiglare layer is formed by aggregation of particles such as cohesive silica; a film in which a surface roughness on a layer is formed by adding an organic filler in a resin; and a film in which a surface roughness is transferred on the surface of a layer by laminating a film with projections and depressions.
Such conventional antiglare films all have a light diffusion function and an antiglare function due to the effect of a surface shape of the antiglare layer. In order to improve such antiglare properties, a surface roughness needs to be steep or dense. However, if a surface roughness becomes steep or dense, the haze (haze value) of a coating increases to cause white muddiness, which results in a decrease in contrast of display images.
Further, the conventional antiglare films generate twinkling brightness, so-called screen scintillation, on the surface, which also results in deterioration of visibility of a display screen. Screen scintillation is a phenomenon that fine brightness ununiformity appears on a screen surface when an image display device is turned on and transmitted light from the rear face reaches the screen, and the position of the brightness uniformity seems to be shifting when a viewer changes the viewing angle. The phenomenon is particularly noticeable in the case of full face white display and full face green display.
For example, an antiglare film including a hard coating layer and an antiglare layer laminated to each other is known (refer to, for example, Patent Literature 3). Such a layered anti-reflection film can suppress occurrence of screen scintillation and white muddiness while maintaining hard coating properties and antiglare properties in such a way that fine projections and depressions on the surface of the antiglare layer are smoothed using the same resin as a hard coating layer, thereby smoothing undulation, reducing fine projections and depressions, and making the heights of projections lower than those of conventional ones. However, in such a case, the film thickness increases to 10 μm or more, and thus, such a film fails to sufficiently satisfy the recent requirement for reducing film thickness.
In cases where a surface roughness on a layer is formed using organic fine particles or inorganic fine particles, and an antiglare film is made thin, the fine particles are often aggregated in the height direction, so that the height of the surface roughness increases and screen scintillation or white muddiness is caused. In order to deal with such a defective point, the average particle size of the organic fine particles or inorganic fine particles is made small to lower the height of the surface roughness. In this case, however, the height tends to be rather too much lowered, and the antiglare properties are likely to deteriorate. Therefore, high quality products cannot be stably obtained.
Accordingly, it has been desired to provide an antiglare film including an antiglare single layer with a smooth surface roughness and excellent in contrast in a dark room, capable of sufficiently suppressing screen scintillation and white muddiness in a bright room while maintaining hard coating properties and antiglare properties.